1. Field of the Invention
The present invention relates to a method for measuring a vibration characteristic of a cantilever installed in a scanning probe microscope.
2. Description of the Related Art
A scanning probe microscope causes a probe which is attached to a tip end of a cantilever to approach a surface of a sample or brings the probe into contact with the surface of the sample and then measures a surface shape of the sample. As measurement modes of the scanning probe microscope, (1) a contact mode and (2) a so-called dynamic force mode (DFM measurement mode has been known. In the contact mode, an interatomic force between a probe and a sample is held to be constant and a surface shape of the sample is measured. In the DFM measurement mode, a cantilever is forced to vibrate at a frequency near a resonant frequency by using a piezoelectric element and a shape of the sample is measured by using attenuation of amplitude of the probe due to an intermittent contact between the probe and the sample when the probe is caused to approach the sample (see, for example, JP-H7(1995)-174767).
However, each cantilever has a subtly different shape and a different index which has an influence on a resonant frequency which is the vibration characteristic of the cantilever and measurement sensitivity referred to as a Q value. For this reason, in the DFM measurement mode or a non-contact measurement mode, there may be necessary that when measurement is performed, the above-described resonant frequency and Q value are measured in advance and measurement of a sample is performed based on these values.
In the related art, a resonant frequency and a Q value are measured as illustrated in FIG. 5. That is, in a state where a cantilever is separated from a sample, vibration intensity is held to be constant and amplitude and a Q-curve (frequency-amplitude characteristic) C1 illustrated in FIG. 5 are measured while vibration is applied at a predetermined sweep speed in a frequency range including the resonant frequency. Accordingly, a resonant frequency f1 and a Q value can be measured by analyzing a waveform of the Q-curve C1. That is, a frequency f1 at a peak position in the Q-curve C1 corresponds to the resonant frequency and the Q value is measured by using Q value=f1/Fw (Fw: a half width of the Q-curve (FWHM)). The Q value is an index indicating viscosity of the cantilever. The Q value is controlled by detecting a speed signal from vibration of the cantilever when a sample is measured and adding the detected speed signal to a vibration applying signal and thus it is possible to obtain higher resolution than that in the related art.
In addition, regarding measurement of a resonant frequency, a frequency sweep signal for reciprocating is generated for a short time, a frequency at the maximum of amplitude in each of an approaching motion and a retracting motion is measured, and the median value of the frequency is detected as a resonant frequency. Accordingly, it is possible to measure a resonant frequency with high accuracy for a very short time (see, for example, JP-A-2012-202841).
Meanwhile, the Q-curve illustrated in FIG. 5 is measured by obtaining a vibration amplitude when a vibration frequency is swept (changed). However, an optimal sweep speed (sweep time) varies depending on the Q value. For example, in FIG. 5, if the sweep speed is slow, a correct Q-curve C1 is obtained, but if the sweep speed is excessively fast, a Q-curve C2 is obtained. A waveform of the Q-curve is changed and thus the correct Q value and a correct resonant frequency f1 are not obtained. That is, it may be necessary to measure a Q-curve in order to obtain a Q value and a resonant frequency f1, but there may be a problem in that a measurement condition for the Q-curve itself depends on the Q value. Furthermore, the resonant frequency f1 corresponds to a peak value of the Q-curve, but depends on the sweep speed. Thus, a peak of the amplitude is shifted in a sweeping direction and becomes a value different from an original value.
Accordingly, in the related art, for example, the Q-curve is measured by determining the sweep speed based on experience or repeating to change the sweep speed and to perform measurement over and over again, or the sweep speed is delayed to measure the Q-curve for a long time. Since it is unclear whether or not the obtained Q value is a correct value based on the optimal sweep speed, measurement is performed with a setting which is shifted from a setting of an optimal measurement condition in many cases.
In this manner, there may be problems as follows. If an accurate Q value is intended to be obtained, a time of measuring the Q-curve becomes long and measurement efficiency may be degraded. If a measurement time of Q-curve is short, the Q value becomes inaccurate and it may become impossible even to determine whether or not the obtained Q value is the correct value.